Blood vessel occlusions are commonly treated by mechanically enhancing blood flow in the affected vessels, such as by employing a stent. Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of affected vessels. Typically stents are capable of being compressed, so that they can be inserted through small lumens via catheters, and then expanded to a larger diameter once they are at the desired location. Examples in the patent literature disclosing stents include U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor.
Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. Local delivery of a therapeutic substance is a preferred method of treatment because the substance is concentrated at a specific site and thus smaller total levels of medication can be administered in comparison to systemic dosages that often produce adverse or even toxic side effects for the patient.
One method of medicating a stent involves the use of a polymeric carrier coated onto the surface of the stent. A composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend can be applied to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer.
The dipping or spraying of the composition onto the stent can result in a complete coverage of all stent surfaces, i.e., both luminal (inner) and abluminal (outer) surfaces, with a coating. However, from a therapeutic standpoint, drugs need only be released from the abluminal stent surface, and possibly the sidewalls. Moreover, having a coating on the luminal surface of the stent can have a detrimental impact on the stent's deliverability as well as the coating's mechanical integrity. A polymeric coating can increase the coefficient of friction between the stent and the delivery balloon. Additionally, some polymers have a “sticky” or “tacky” nature. If the polymeric material either increases the coefficient of friction or adherers to the catheter balloon, the effective release of the stent from the balloon upon deflation can be compromised. Severe coating damage at the luminal side of the stent may occur post-deployment, which can result in a thrombogenic surface. Accordingly, there is a need to eliminate or minimize the amount of coating that is applied to the inner surface of the stent. Reducing or eliminating the polymer from the stent luminal surface also means a reduction in total polymer load, which will minimize the material-vessel interaction and is therefore a desirable goal for optimizing long-term biocompatibility of the device.
A method for preventing the composition from being applied to the inner surface of the stent is by placing the stent over a mandrel that fittingly mates within the inner diameter of the stent. A tubing can be inserted within the stent such that the outer surface of the tubing is in contact with the inner surface of the stent. With the use of such mandrels, some incidental composition can seep into the gaps or spaces between the surfaces of the mandrel and the stent, especially if the coating composition includes high surface tension (or low wettability) solvents. Moreover, a tubular mandrel that makes contact with the inner surface of the stent can cause coating defects. A high degree of surface contact between the stent and the supporting apparatus can provide regions in which the liquid composition can flow, wick, and/or collect as the composition is applied to the stent. As the solvent evaporates, the excess composition hardens to form excess coating at and around the contact points between the stent and the support apparatus, which may prevent removal of the stent from the supporting apparatus. Further, upon removal of the coated stent from the support apparatus, the excess coating may stick to the apparatus, thereby removing some of the coating from the stent and leaving bare areas. In some situations, the excess coating may stick to the stent, thereby leaving excess coating composition as clumps or pools on the struts or webbing between the struts. Accordingly, there is a tradeoff when the inner surface of the stent is masked in that coating defects such as webbing, pools and/or clumps can be formed on the stent.
In addition to the above mentioned drawbacks, other disadvantages associated with dip and spray coating methods include lack of uniformity of the produced coating as well as product waste. The intricate geometry of the stent presents a great degree of challenges for applying a coating material on a stent. Dip coating application tends to provide uneven coatings and droplet agglomeration caused by spray atomization process can produce uneven thickness profiles. Moreover, a very low percentage of the coating solution that is sprayed to coat the stent is actually deposited on the surfaces of the device. A majority of the sprayed solution is wasted in both application methods.
To achieve better coating uniformity and less waste, electrostatic coating deposition has been proposed. Examples in patent literature covering electrostatic deposition include U.S. Pat. Nos. 5,824,049 and 6,096,070. Briefly, referring to FIG. 1, for electro-deposition or electrostatic spraying, a stent 100 is grounded and gas is used to atomize the coating solution into droplets 110 as the coating solution is discharged out from a nozzle 120. The droplets 110 are then electrically charged by passing through an electrical field created by a ring electrode 130 which is in electrical communication with a voltage source 140. The charged particles are attracted to the grounded metallic stent. An alternative design for coating a stent with an electrically charged solution is disclosed by U.S. Pat. No. 6,669,980. U.S. Pat. No. 6,669,980 teaches a chamber that that contains a coating formulation that is connected to a nozzle apparatus. The coating formulation in the chamber is electrically charged. Droplets of electrically charged coating formulation are created and dispensed through the nozzle and are deposited on the grounded stent. Stents coated with electrostatic techniques have many advantages over dipping and spraying methodology, including, but not limited to, improved transfer efficiency (reduction of drug and/or polymer waste), high drug recovery on the stent due to elimination of re-bounce of the coating solution off of the stent, better coating uniformity, and a faster coating process. Formation of a coating layer on the inner surface of the stent is not, however, eliminated with the used of electrostatic deposition. With the use of mandrels that ground the stent and provide for a tight fit between the stent and the mandrel, formation of coating defects such as webbing, pooling and clumping remain a problem. If a space is provided between the mandrel and the stent, such that there is only minimal contact to ground the stent, the spraying can still penetrated into the gaps between the stent struts and coat the inner surface of the stent. Conventional stent geometry does not provide for a good Faraday cage due to the interspace between the struts of the stent. As illustrated by FIG. 2, electric field lines can penetrated into the opening between the struts and deposit a coating on the inner surface of the stent. This is known as the “wrap around” effect. Charged particles are not only disposed on the outer surface of the stent, but also are wrapped around each strut and are attracted to the inner surface of the stent.
Accordingly, what is needed is a stent and method that allows for electro-deposition or electrostatic spraying of a stent while eliminating or minimizing the wrap around effect.